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(Inauguration speeches, Agricultural University, Wageningen, 1976, unpublished)
point out that selectivity can also be improved by choosing the optimum
time, place, and method of application; by improving the formulation;
and by mixing a pesticide with other chemicals.
In certain cases, higher pesticidal activity can also be achieved by removing
the less-active optical isomers (e.g., in pyrethroids) or less-active and nonactive
components (e.g., removing 1,2-dichloropropane from a mixture of
chlorinated C3 hydrocarbons that is sold as a soil fumigant).
Progress in improving the selectivity and efficacy of pesticides is demonstrated
by the fact that application rates have decreased several orders of
magnitude since the beginning of the century (Fig. 3). Lower application rates
also mean less exposure for those applying the pesticides in the field. Insofar
as the increase in pesticidal activity has been matched by an increase in target
species selectivity, an overall reduction in the risk to nontarget organisms
(including humans) has been achieved.
Although the differences in toxicity between the target species and humans is
of paramount importance, other factors can influence the health hazard. As
skin contamination is the most important route of exposure during pesticide
application, the degree of skin penetration of a substance has a significant
bearing on the risk of intoxication. The amount of active ingredient used, the
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Fungicides
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148
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1900 1940 1980
Fig. 3. Application rates (logarithmic scale) for crop protection chemicals plotted
against the date of their introduction to illustrate the progressive increase in activity.
Rates are representative values from the range employed in commercial practice for
each compound (I.J. Graham-Bryce, Royal Society, London, UK, 1981).